Polypropylene resins have conventionally been used in a wide variety of fields because they are relatively inexpensive and have excellent properties. However, there has been a desire for an improvement in impact resistance, in particular, low-temperature impact resistance. Many techniques for eliminating this problem have been proposed so far. A generally employed technique is to produce a propylene block copolymer by firstly forming a propylene homopolymer ingredient and then incorporating an ethylene-propylene random copolymer ingredient. Compared to propylene polymers, propylene block copolymers are inferior in rigidity, hardness, and heat resistance although improved in impact resistance.
Among techniques which have been proposed for mitigating these drawbacks is a method in which the ratio of the melt flow rate of a first-stage polymer to that of a second-stage polymer is regulated as described in Unexamined Published Japanese Patent Application No. 5-117342. This method is in extensive use in various industrial fields such as the fields of motor vehicles and domestic electrical appliances.
However, even the above-described technique according to Unexamined Published Japanese Patent Application No. 5-117342 has still had an insufficient balance among rigidity, toughness, impact resistance, etc.
An object of the present invention is to provide a polypropylene/propylene-ethylene copolymer composition excellent in rigidity, toughness, and impact strength and to further provide a process for producing the same.
In view of the problems described above, the present inventors made intensive studies in order to obtain an improved balance between rigidity and toughness/impact resistance in propylene-ethylene block copolymers. As a result, it has been found that a polypropylene/propylene-ethylene copolymer composition excellent in rigidity, toughness, and impact strength is obtained by yielding a propylene-ethylene copolymer composition through two-stage polymerization in such a manner that a polypropylene having a tie molecule volume content (.beta.) of 1.38% or higher is produced as a product of the first stage and a propylene-ethylene copolymer is yielded, in the second stage, around the polypropylene yielded in the first stage. The present invention has thus been completed.